As is well known, most of the commercial acrylonitrile is produced with the Sohio Process from propylene by heterogeneous catalytic ammoxidation of propylene in the vapor phase with ammonia, air and steam. For example see U.S. Pat. Nos. 3,222,422 in the name of L. A. Cohen; 3,278,642 and 3,346,520 both in the name of L. Lee; 3,442,981 in the name of O. L. Stafford, D. V. Wing and D. E. Stolsmark; and 3,509,238 in the name of N. E. Aubery and M. B. Jastrzebeski.
In a commercial acrylonitrile system utilizing this process, the reactor feeds are propylene, ammonia and compressed air. The propylene and ammonia are vaporized, then combined with the air and fed to a fluidized bed catalytic reactor. Precise ratios of the three feeds are maintained for optimum yield. The catalyst in the reactor vessel is in the form of particles, which are maintained in a turbulent fluid state by the velocity of gaseous flow through the bed.
Propylene, ammonia and oxygen mix together in the reactor and oxidation of propylene in the presence of ammonia takes place on the surface of the fluidized catalyst. A set of complex exothermic reactions takes place, thereby forming the following products: acrylonitrile, hydrogen cyanide, carbon dioxide, carbon monoxide, acetonitrile, acrolein, acrylic acid, water, other higher nitrites, aldehydes, ketones, acetic acid and a number of miscellaneous unknown organic compounds. Conversion of the three feeds is less than 100 percent, thus unreacted propylene, ammonia, oxygen and nitrogen are contained in the reactor effluent gas. The source of propylene typically contains a small amount of propane and some heavier hydrocarbon compounds which most of which are purged from the process unreacted. A portion of the heat of the exothermic reaction is removed by sets of steam coils which generate and superheat waste steam at approximately 600 psig for process uses such as heat input for 9 distillations in the products recovery and purification section of the process. Reactor effluent gas passes through cyclones, which remove catalyst fines from the gas. The gas is then further cooled in a reactor effluent cooler, which is comprised of a shell and tube exchanger using boiler feed-water as the cooling source.
As the gas leaves the reactor effluent cooler, it then enters a quench column. The quench column cools the reactor effluent by contacting it with a recirculating water stream. Most of the water vapor and small amounts of organic vapors in the reactor effluent are condensed in the quench column. The quench column bottoms are cooled and circulated back to the quench column. The quench column can contain internal trays or packing to provide intimate contact of upflowing gas with downflowing water. Sulfuric acid is injected into the recirculating quench water to neutralize unreacted ammonia in the reactor effluent. The excess quench water is roughly equal to the amount of water produced by the reactor and is fed to the wastewater column where acrylonitrile and hydrogen cyanide are recovered. Wastewater column bottoms are cooled and neutralized, mixed with other plant waste streams, clarified and injected into the wastewater injection well. The quench column effluent gas is then directed to an absorber where chilled water is used to obtain an aqueous solution of acrylonitrile, hydrogen cyanide and other organics from the gas.
The aqueous solution from the absorber is fed to a recovery column where acrylonitrile and hydrogen cyanide are taken overhead. A portion of the bottoms from the recovery column is cooled and recycled to the absorption step. This recycle contains both inorganic and organic compounds in the form of monomers, oligomers, prepolymers, and polymers in various combinations. Acrylonitrile, hydrogen cyanide and optionally acetonitrile products are then purified using a series of distillations and phase separations. A first column (heads column) removes hydrogen cyanide, and at last column (acrylonitrile product column) takes a pure acrylonitrile monomer product from a side-draw near the top of the column. High-boiling organic compounds are rejected from the product column bottoms.
Acrylonitrile can polymerize in the quench column. More specifically, as the reactor effluent gas is passed through the quench column, a portion of the acrylonitrile contained in the gas polymerizes and is absorbed into the recirculating quench water. The amount of acrylonitrile that polymerizes in the quench column represents an undesirable net product loss for the acrylonitrile plant. For example, in an uninhibited quench column, between about 2 to 5 percent of the total acrylonitrile produced by the reactor is lost due to polymerization in the quench column.
Several methods are known to reduce losses of acrylonitrile by polymerization and other side reactions, which involve treating the recirculating quench water. For example see U.S. Pat. Nos. 3,691,226; 4,720,566; 5,869,730; 5,895,822 and 6,238,574, which patents are incorporated herein by reference.
U.S. Pat. No. 4,720,566, in the name of John F. Martin, describes methods and compositions for inhibiting acrylonitrile polymerization in quench columns of systems producing acrylonitrile with a combination of (a) a hydroxylamine having two alkyl groups, and (b) a para-phenylenediamine with a substituent phenyl group or unsubstituted para-phenylenediamine.
Unfortunately, under operating conditions, acrylonitrile can also polymerize in the recovery and purification sections to from solid deposits which interfere with operation of equipment, contribute to an undesirable net production loss and reduction in production rates, and with time lead to costly shutdowns.
Such polymers, oligomers, and prepolymers, in various combinations, foul heat exchange surfaces of the heat exchangers used to maintain operating conditions for separation in the distillation columns and other process equipment. Fouled heat exchange surfaces reduce the coefficient of heat transfer thereby increasing the amount of heat transfer medium which must be used to realize the required amount of heating and/or cooling obtained on clean surfaces. Eventually, the heat exchanger must be manually cleaned with potential exposure of personnel to hazardous chemicals.
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for recovery and refining of valuable nitrogen-containing organic compounds formed by catalytic oxidation of least one feed compound selected from the group consisting of propane, propylene, isobutane and isobutylene in the presence of ammonia.
Improved processes would utilize a preselected class of polymerization inhibiting compositions effective under operating conditions during fractional distillations of aqueous solutions comprising the unsaturated mononitrile products.
Advantageously, members of such a class of inhibiting compositions would be effectively separated by the fractional distillation of the purified products.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.